2.3 The Goldilocks planet

So why are we describing all this cosmology in a Chapter about
the evolutionary origins of the fungi? Our reason is that all this cosmology was
essential to make the Earth-Moon binary system such a very special habitat for
life. A habitat that is unique in its:

Orbital position: not too hot and not too cold (giving
the planet plenty of liquid water).

Nature as a binary planetary system (giving the planet
ocean tides to add to its seasonal climate).

As mentioned, our orbital position is crucial to maintaining
liquid water on the surface of the planet. You only have to think of our
seasonal changes to realise how exact that positioning must be. The Earth's axis
is tipped over about 23.5° from the vertical and it’s our annual orbital motion
around that which causes our seasons. We're European northern hemisphere people,
and used to wintry ice and snow from December to February and warm summers from
June to September. But these seasons only result from the old planet being
angled slightly towards the sun in summer, or away from the sun in winter. Just
that small rotational shift towards or away from the sun is sufficient to
generate an enormous temperature differential. The highest recorded temperature
in Europe (in Seville, Spain) is 50˚C; compared to the lowest recorded European
temperature of -55˚ C (at Ust-Shchugor, Russia). You will appreciate that if
Earth’s orbit was very slightly closer to the Sun, let’s say by about one Earth-radius,
surface temperatures would be intolerably high all the time.
Alternatively, if the planet was a similar distance further from the Sun, it
would be in permanent deep freeze.

That highly improbable impact between proto-Earth and Theia not
only topped-up the Earth’s liquid iron core but it imparted increased rotation
to both planet and core. The molten core spins now, and the spinning iron
generates our magnetic field, the magnetosphere, producing a
magnetic field strong enough to protect Earth from the solar wind.

The solar wind is a stream of charged particles
ejected from the upper atmosphere of the Sun and consists mostly of high-energy
electrons and protons. Mars is unprotected and the solar wind has stripped away
up to a third of its original atmosphere. Even the dense atmosphere of Venus is
being eroded by the solar wind, so much so that space probes have discovered a
comet-like tail
of drifting atmosphere that stretches back
from Venus to the orbit of the Earth.

But Earth is safe, protected from the solar wind by its
magnetic field, which deflects charged particles but also serves as an
electromagnetic energy transmission line to the Earth's upper atmosphere and
ionosphere through the Aurora Borealis
in the north, and Aurora Australis
in the south. By preventing the solar wind stripping
away the ozone layer our magnetosphere also protects the surface of Earth from
biologically harmful ultraviolet (UV) radiation emitted by the
Sun as well as the high-energy particles of the solar wind.

This is crucially important to biology; UV wavelengths shorter
than 280 nm (also known as ‘germicidal UV’ or UV-C) is entirely
screened out by ozone at an altitude
of around 35 km.For radiation with a
wavelength of 290 nm (UV-B, which is most damaging to DNA), the intensity at
Earth’s surface is only 10-8 that at the top of the atmosphere,
thanks mainly to high-altitude ozone.

And that leaves the contribution of our enormous Moon,
by far the largest satellite in the Solar System (in
proportion to its planet) and constructed from the rocky remnants
of the Theia-Earth impact (Fig. 3). The presence of the Moon gradually reduced
the rotation rate
of the Earth to a level that reduces temperature
variations on the Earth’s surface to life-supporting limits. But it also
stabilises our axial tilt, which ensures annual seasons and
challenging environments to drive evolution. Similarly, the Moon generates
tidal effects
in both rocks and water, and the latter also produce variable shoreline
environments that spur chemical and biological evolution(Zalasiewicz & Williams, 2013).

Resources Box

The story of Planet Earth

For more information about the origins of the Earth we
recommend a collection of publications and websites.

CLICK HERE to listen to
Eric Idle's Galaxy Song (from
Monty Python’s The Meaning of Life, 1983). Note that this track
is in Windows Media Audio (WMA) format.
You may need to download and install
a player app.